Nonvolatile memory cells such as those of the floating gate type are well known in the art. In a floating gate non-volatile memory cell, the cell is constructed from a semiconductor substrate of a first conductivity type. The substrate has a first and a second region of a second conductivity type spaced apart from one another with a channel region there between. A floating gate is insulated and spaced apart from at least a portion of the channel region. Charges are placed on the floating gate by a variety of mechanisms including but not limited to hot electron injection, Fowler-Nordheim tunneling, etc. Charges are removed from the floating gate to either a control gate, or either the first or second regions, or the channel itself. Such removal can occur by Fowler-Nordheim tunneling.
Typically, non-volatile memory floating gate cells have been of two types: stacked gate type or split gate type. In a split gate type, a floating gate is positioned over only a first portion of the channel region and controls the conduction of charges between the first region and the second region only in that first portion of the channel region. The control gate, which is separate and apart from the floating gate, controls the second portion of the channel region. To operate, the control gate must be “turned on” thereby permitting electron flow to occur in the channel region in the second portion. The state of charges accumulate on the floating gate control the conduction of the channel region in the first portion.
In a stacked gate non-volatile memory cell floating gate, the control gate is “stacked” on top of the floating gate. The floating gate is spaced apart and adjacent to the entire channel region and the state of charges retained on the floating gate control the conduction of the entire channel region.
A dynamically tunable resistor or capacitor is also well known in the art. Referring to FIG. 1 there is shown a dynamically tunable resistor 10 of the prior art. In the tunable resistor 10 of the prior art, a plurality of resistors (R0-R6) are connected in series. The resistor R0 has one end connected to a voltage source VCC and another end connected to the output node Rout. The resistors R1-R6 are all connected in series to the node Rout. The other end of the series of resistors R1-R6 is connected to ground. Each of the resistors has one node connected to a switch Sx which connects the resistor to ground thereby bypassing all of the other resistors. Thus, by selectively switching the switches S1-S5, various amounts of resistance can be placed in series with the resistor R0 thereby altering the resistance at the node Rout.
A dynamically tunable capacitor 20 of the prior art is shown in FIG. 2. A capacitor C0 is connected between VCC and ground. The node Cout at VCC provides the output of the variable capacitor 20. The node Cout is also connected to a plurality of capacitor C1, C2, C3 and C4. Each of the capacitors C1-C4 is connected through a switch Sx in parallel between VCC and ground. Thus, the addition of each capacitor Cx placed in parallel with capacitor C0 changes the capacitance at Cout. By varying the switches S1-S4, different amounts of capacitance can be placed in parallel with the capacitor C0 thereby altering the capacitance at the node Cout.
Although the variable resistor 10 and variable capacitor 20 of the prior art are satisfactory for their use, because these devices are made from integrated circuits, there are certain slowly degrading features of the integrated circuit that will cause them to drift away from originally designed optimized value for the resistance or capacitance. Although the circuit may still function, the quality may degrade from the optimized point during a prolonged period of use such as ten years of a life of a system. In addition, in particular applications such as radio frequency where the frequency of operation is high, such as 1 GHz-1000 GHz, such RF applications require very precisely tuned resistors and precisely tuned capacitors which do not vary or drift from originally designed values as the device is placed in operation. Thus, one object of the present invention is to provide a dynamically tunable, i.e. in situ variable resistor or variable capacitor that can be changed as the operation of the integrated circuit varies over the lifetime of its usage.